You could do this by creating a custom WP_Query
and then counting it.
$args = array(
'author' => 1,
'cat' => 5,
);
$my_query = new WP_Query( $args );
$my_count = $my_query->post_count;
Just change 1
and 5
to the Author ID and Category ID respectively.
Alternatively you can use the category slug or author’s nice name (NOT name!) as well:
$args = array(
'author_name' => 'bob',
'category_name' => 'editorial',
};